Technical Field
The present disclosure relates to batteries of portable electronic appliances, such as the batteries of laptops, digital tablets and mobile telephones such as smartphones.
Description of the Related Art
Currently, a major drawback of appliances such as mobile telephones and digital tablets lies in their short battery time. Furthermore, as it is possible to install many very diverse applications with varying degrees of complexity in these appliances, it is difficult for manufacturers to anticipate any pattern of use of them and thus to estimate the extent to which the battery will be used.
Some energy gauging devices do exist and are installed in these appliances to provide an estimation of the state of charge of the battery, for example in percentage compared to minimum and maximum charges. However, such devices do not generally take into account any drifts gradually occurring in operating parameters of a battery over the charge and discharge cycles. The result is that the estimated state of charge provided by these devices becomes less and less representative of an actual state of charge over time.
Generally, the energy gauging devices use information such as the current and the voltage of the battery to estimate the state of charge of the battery. The simplest of such devices are based on an open-circuit measurement of the voltage, e.g., when the battery is not connected to any charge and is not supplying any current. The open-circuit voltage measurement gives a good indication of the state of charge of a battery. However, when this measurement is taken while the battery is supplying current or has just supplied current, the state of charge estimated on the basis of such a measurement can contain a significant error. This error is due to variations in the internal impedance and physico-chemical time constants of the battery.
Certain energy gauging devices implement a Coulomb counter which measures the current and integrates it over time to give a measurement of the charge respectively supplied or received by the battery during a discharge or a charge. Such a measurement obtains excellent linearity, and therefore obtains good short-term accuracy. However, this use requires an initial value to be determined, as the Coulomb counter merely supplies charge variation information rather than an absolute value of charge quantity supplied. Coulomb counters also undergo current measurement drifts that build up over time.
Certain energy gauging devices use modeling of the electrical behavior of the battery. The simplest and most frequently used model consists of an open-circuit voltage source based on a state of charge, and of a constant series impedance. This model, which differs from one type of battery to another, can be obtained from a characterization of the battery. The devices based on such a model enable the current to be predicted and the state of charge of the battery to be estimated from a battery voltage measurement with better robustness and accuracy than devices based solely on a voltage measurement. In particular, these devices obtain good long-term accuracy while avoiding drifts. However, these devices are quite ineffective to monitor the short-term changes in the state of charge of the battery, due to insufficient accuracy in the definition of the model parameters such as the internal impedance of the battery, and due to the use of open-circuit voltage measurements. While the variations in the internal capacity of a battery can be easily assessed and taken into account, the same is not true of the variations in the internal resistance of the battery. Now, an incorrect assessment of the variations in the internal resistance can generate an instability that could cause jumps and oscillations in the estimation of the state of charge of a battery.
Certain devices combine a current integration through a Coulomb counter and a voltage measurement with or without a model to correct the state of charge estimation obtained from the current measurement. However, the corrections are made by discrete steps, which generates jumps or oscillations in the estimation of the state of charge. As such jumps or oscillations are not acceptable for the user, the value of the state of charge is filtered. Now, such filtering inevitably generates a delay in the variations in the state-of-charge value and thus affects the accuracy of this value. In addition, the adjustment of the filter parameters, particularly based on the battery impedance, proves tricky due to the multiple types of battery and types of appliances powered by a battery, and due to the difficulty estimating the internal resistance of the battery.
To take the drifts of the operating parameters into account throughout the lifetime of a battery, complex algorithms have been developed, in particular combining different methods to overcome the limitations of each method. Therefore, energy gauging devices have recently been developed which comprise a Coulomb counter and a voltage measurement and take the internal resistance and the capacity of the battery into account.
Another example of a complex algorithm is given in U.S. Pat. No. 8,103,485 which describes an energy gauging device implementing a double extended Kalman filter and a simultaneous acquisition of the current and the voltage of the battery. This device offers the main advantage of continuously correcting the estimation of the state of charge of a battery based on a complex model of the battery.
However, energy gauging devices implementing complex algorithms require significant calculation resources, and several analog-digital converters to simultaneously measure the voltage, the current and possibly the temperature of the battery. These devices also require complex management of software configurations to take all the combinations of appliances and types of battery currently marketed into account. Some of these devices must also manage calibration steps.
Energy gauging devices implementing complex algorithms thus prove to be rather ill adapted to implementation in appliances such as smartphones and tablets, for reasons of cost and limited battery life.